The demand for developing ultra-linear microwave transmitter, supporting high crest factor signals, increases rapidly, for example by the definition of the third generation (hereinafter “3G”) of mobile radio standards.
The demanding adjacent channel power ratio (hereinafter “ACPR”) requirements of these systems, i.e., W-CDMA or cdma2000, present a critical issue for transmitter designers if both ultra-linearity and high power efficiency must be met. In fact, the degradation of linearity becomes significant as the power amplifier (hereinafter “PA”) operates close to saturation where both high power efficiency and high output power emission are achieved. Therefore, for different stimulus levels driving the amplifiers and for a given ACPR specification, the trade-off between power efficiency and linearity impose an operating point with poor power efficiency. In this case, linearization techniques become the only possible way to recuperate the linearity and to allow optimal trade-off.
Various linearization methods have been reported and are derived, by any measure, from three main types named:
Feed-forward (R. Meyer, R. Eschenbach and W. Edgerley, Jr. “A wide-Band Feedforward Amplifier”, IEEE J. of Solid-State Circuits, vol. sc-9, no. 6, pp. 442–428, December 1974), which includes an open loop configuration, can handle a multicarrier signal but can not easily be controlled against the effects of drift. Moreover, their low power efficiency make it suitable in base station only. A good analysis of adaptation behavior has been presented in J. Cavers, “Adaptation Behavior of a Feedforward Amplifier Linearizer”, IEEE Transactions on Vehicular Technology, vol. 44, no. 1, pp. 31–40, February 1995;
Feedback (A. Bateman & D. Haines, “Direct Conversion Transceiver Design for Compact Low-Cost Portable Mobile Radio Terminals” IEEE Conf. pp. 57–58, 1989), which presents an excellent reduction of out-of-band emissions, is relatively easy to implement. However, stability requirement limits its bandwidth because of its critical dependence on the loop delay; and
Predistortion (N. Imai, T. Nojima and T. Murase, “Novel Linearizer Using Balanced Circulators and Its Application to Multilevel Digital Radio Systems”, IEEE Transactions on Microwave Theory and Techniques, vol. 37, no. 8, pp. 1237–1243, August 1989), this technique has historically been the most common method in analog implementation. This method uses a nonlinear element which precedes the device to be compensated, its gain-expansion characteristic cancels the gain compression of the amplifier. Like feed-forward, it has an open loop configuration and therefore is very sensitive to drifts.
The predistortion technique has historically been the most common method in analog implementation. Now, this technique is well suited to digital implementation, for example by integrating a DSP chip to handle high-speed arithmetic. In this way, important experimental results have been presented in the following papers, demonstrating the capability in reducing the spectral spreading and how adaptive correction for drift, aging and temperature variation can be achieved using DSP circuits.
[1] Y. Nagata, “Linear Amplification Technique for Digital Mobil”, in Proc. IEEE Veh. Technol. conf., San Francisco, Calif., 1989, pp. 159–164.
[2] M. Faulkner, T. Mattsson and W. Yates, “Adaptive Linearization Using Predistortion” in Proc. 40th IEEE Veh. Technol. Conf. 1990. pp. 35–40.
[3] A. S. Wright and Willem G. Durtled, “Experimental Performance of an Adaptive Digital Linearized Power Amplifiers”, IEEE Transactions on Vehicular Technology, vol. 41, no 4, pp. 395–400, November 1992.
[4] J. Cavers, “Amplifier Linearization Using a Digital predistorter with Fast Adaptation and Low Memory Requirement”, IEEE Transaction on Vehicular Technology, vol. 39, no 4, pp. 374–382, November 1990.
[5] E. G. Jeckeln, F. M. Ghannouchi and Mohamad Sawan, “Adaptive Digital Predistorter for Power Amplifiers with Real Time Modeling of Memoryless Complex Gains”, IEEE MTT-S 1996 International Microwave Symposium, San Francisco, Calif., June 1996.
Although the above-mentioned technique is powerful due to its digital operating principles, it presents certain inflexibility in the sense that; it is suitable only when the baseboard signal is acceded directly before the up-conversion. In most cases, linearizer designers have no access to baseband signal; hence, they found themselves confined to use traditional Radio-frequency (hereinafter “RF”) analog predistortion techniques. In this case, it is more difficult to meet severe ACPR for a high crest factor's signals specifications while operating not in far back-off regions.
The RF-based predistorter proposed by Stapelton et al. (S. P. Stapelton and F. C. Cotescu, “An Adaptative Predistorter for a Power Amplifier Based on Adjacent Channel Emissions”, IEEE Transactions of Vehicular Technology, vol. 41, no 1, pp 49–56, February 1992) offers an interesting alternative. It includes a complex gain tuning circuit that controls the amplitude and phase of the RF signal. The baseband environment is confined to optimize two nonlinear work functions by monitoring the ACP-minimization (Adjacent Channel Power) measured by a power detector. Drawbacks of this method are its slow convergence toward the minimum and its sensitivity to the measurement noise.